Make ASSERT_LIVE_IS_ALLOWED() behave as a function

Make ASSERT_LIVE_IS_ALLOWED() behave as a function

David Kastrup helpfully pointed out that this macro expanded into
multiple statements, whereas it looks like a function call.  For
example, something like this:
  if (condition) ASSERT_LIVE_IS_ALLOWED(); else
could fail quite badly.

The "do {...} while (0)" is an idiom that compilers optimize away,
but allows the macro to behave as a function.